Thermal destruction is commonly employed for the destruction of waste, including municipal refuse and industrial waste. Two methods for thermal destruction are plasma gasification (electric) and fossil fuel-fired incineration. In both cases, a high-temperature source is applied, leading either directly to combustion (incineration) or gasification (plasma) followed by combustion. Both methods lead to the formation of an inorganic residue (e.g.: ash, slag) and hot combustion gases. Typical energy recovery efforts focus on extracting heat from the hot exhaust. These efforts can include the use of a boiler to generate steam (the Rankine cycle) or the use of a heat exchanger to generate a hot liquid resource. If the waste being combusted/gasified contains a high level of chlorine (such as when chlorinated plastics are burned), these thermal exchange methods are typically replaced by a rapid liquid quench to prevent the formation of toxic by-products, such as dioxins and furans.
The Rankine cycle is a process by which a fluid, typically water, is cyclically evaporated, expanded through a turbine and condensed. The Rankine cycle is the principle of operation of several common devices, including air conditioning units and refrigerators. This process can also be used to capture the thermal energy generated by a process and convert that energy into mechanical work in a turbine (which can then be converted to electricity using a generator). In typical industrial processes, this is accomplished by generating steam from a hot resource by means of a boiler. However, using a boiler to generate steam from waste heat can be challenging and the application potential is limited to large-scale facilities (this is because the efficiency of the steam cycle increases as the available waste heat load increases). For small-scale waste combustion facilities, such as those that would be installed on marine vessels, the steam cycle is inefficient and impractical. The steam generated would be low to medium pressure, at best. Moreover, there are safety concerns that must be addressed when using a steam cycle, related to the high temperature and pressure conditions of the steam. These concerns can lead to certification difficulties in controlled environments, such as on ships. The steam cycle is also meant to operate on a full design thermal load, given that a steam turbine's turn-down is quite poor: if only part of the thermal load is available to the boiler (such as during startup, shutdown or process fluctuations), lower pressure steam will be generated which may be insufficient to activate the steam turbine. Therefore, the steam cycle's ability to cope with load changes is quite limited.
Rather than convert to electricity using the steam cycle, some opt to use the exhaust's thermal energy to either generate steam for heating purposes, or to yield a hot water resource. Such applications can be useful, depending on the needs of the surrounding processes.
In both cases (generation of steam to yield electricity with a turbine or generation of steam/hot water for use as a process resource), the heat from the exhaust is typically the only thermal energy source considered. However, there is a significant heat load typically lost in other portions of the waste destruction process, particularly for shipboard processes. Indeed, IMO (International Maritime Organization) regulations state that the surface temperature of all shipboard waste destruction components be no more than 60° C. This regulation is typically interpreted as meaning that the process components must be water-cooled down to below 20° C. Such aggressive cooling implies that the cooling water flow rate is high and that the temperature increase of the water is limited to a few degrees. As a result, the cooling water return cannot practically be used as a hot water resource and the thermal energy it contains is wasted. For example, in the case of plasma waste gasification systems, this wasted heat load can represent as much as 20% of the total system energy rating (15% lost to the cooling jackets, 5% lost to the plasma torch). Looking specifically at the plasma torch, up to 35% of the gross power supplied to generate the thermal discharge is wasted as a result of aggressive cooling.
The organic Rankine cycle (ORC) is similar to the traditional Rankine cycle, making use of an organic refrigerant instead of steam to convert thermal energy into electricity. Because of the thermo-physical properties of the refrigerants employed (refrigerant R245fa is typically preferred), the ORC is best suited to recover energy from hot liquid resources with temperatures ranging from 90° C. to 150° C. (as its boiling point is significantly lower than that of water). Much like the regular Rankine cycle, the ORC, schematically illustrated in FIG. 1, is composed of four main parts: an evaporator 1, a turbine generator 2, a condenser 3 and a compressor 4. A hot liquid resource 5 is fed to the evaporator 1 to transfer thermal energy to the refrigerant. The refrigerant evaporates and is conveyed to a turbine generator 2 where it is expanded (the work 6 resulting from this expansion in the turbine is converted to electricity by the generator). The expanded refrigerant is condensed in a second heat exchanger (the condenser 3), through indirect contact with cooling water 7. The compressor 4 then pressurizes the condensed refrigerant and the cycle repeats. The efficiency (portion of the thermal load converted into electricity) for the ORC is in the order of 10%. Commercial ORC units are available from several manufacturers.
Therefore, there is a need in the art for improved technology for the recovery of energy in thermal waste destruction systems.